1. Field of the Invention
The present invention relates to an electrolytic capacitor using a metal having a valvular action, such as aluminum, tantalum, or the like, as the anode, a coating layer formed of an oxide of the valvular metal as the dielectric, and an conducting polymer layer as the cathode. The present invention also relates to a method for manufacturing such an electrolytic capacitor.
2. Prior Art
Customary electrolytic capacitors using a valvular metal, e.g. aluminum, tantalum, are generally fabricated using a valvular metal porous body as an anode element, a coating layer of an oxide of the valvular metal which is formed as the dielectric layer on the porous surfaces and external surface of the porous body, and an liquid electrolyte or inorganic solid electrolyte as the cathode. In addition to the above, a metal collector connecting to the anode and the cathode and finally a casing are formed. As the cathode, for example, an organic solvent including an organic acid or the like is used in the aluminum electrolytic capacitor and manganese dioxide or the like for the tantalum electrolytic capacitor.
High responsiveness at high frequencies has been demanded of electronic parts along with the digitization of circuits. It is therefore required for the electrolytic capacitors to be improved in the high responsiveness at high frequencies by a reduction in electric resistance passing through the capacitor. In this situation, the use of a highly conducting polymer as a solid electrolyte for the cathode has been studied and developed.
The electrolytic capacitor has the structural feature in which an oxide layer is formed on the surface of pores of porous valvular metal as the dielectric layer and the valvular metal remaining as the core is used as the anode. When forming the cathode in the capacitor element, it is therefore necessary to coat the surface of very intricate internal pores of the porous element with a conducting polymer in an efficient manner.
In the case of producing a solid electrolyte for the cathode from the conducting polymer by an electrolytic oxidation polymerization method, the following method is used to coat, with the conducting polymer, the surface of the dielectric layer formed on the pore surfaces of the very intricate porous element in an efficient manner. Specifically, a conducting precoat layer is formed in advance on the surface of the insulating dielectric layer. An electrode used for electrolytic oxidation polymerization is allowed to be in contact with the surface of the precoat layer. A solution containing a monomer to be polymerized into an conducting polymer is introduced. An conducting polymer layer is formed by polymerization on the entire surface of the dielectric using the precoat layer as the anode. Then, the electrode for electrolytic oxidation polymerization is removed. On the other hand, as the chemical oxidation polymerization method is utilized, a monomer is allowed to be in contact with an oxidizer capable of oxidizing and polymerizing the monomer on the coating of the dielectric layer to form an conducting polymer layer on the entire surface of the dielectric layer.
In the case where the cathode electrolyte is a solid, a method has been adopted in which an conducting adhesive layers consisting of a carbon paste layer and particulate silver paste layer is interposed between the solid electrolyte layer (manganese dioxide layer or conducting polymer layer) and a current collector for cathode to combine the electrolyte layer with the collector metal.
Such a method is disclosed in Japanese Patent Publication JP-A 6-168,855, in which the current collector for cathode is disposed close to the periphery of the valvular metal element. In case of, for example, a aluminum-laminating capacitor, the current collector for cathode is disposed close to any one or all of the side faces and upper and lower faces of the laminated body as shown in FIG. 13.
There have been used a method of disposing the cathode collectors between the aluminum layers in the aluminum laminated capacitors. Japanese Patent Publication JP-A 4-306,427 discloses a method of joining the cathode collector directly to the conducting polymer layer as a anode.
The aforementioned capacitors having such a structure can only insufficiently reduce in impedance even if it uses a highly conducting polymer as the cathode and hence poses the problem of low responsiveness at high frequencies. This is because various layers are interposed to combine the current collector for cathode with the conducting polymer and also because the surface area of the current collector for cathode is small so that the surface contact resistance cannot be reduced. This is because the natural oxide coating is formed on the surface of the metal collector, causing the surface not to reduce in contact resistance.